Project Summary/Abstract The ultimate goal of this K99/R00 Pathway to Independence Award is to facilitate the transition to a career as an independent cancer research scientist. This goal will be accomplished by providing the opportunity to continue training in cancer biology, acquire expertise in a variety of novel techniques, and generate a body of data that will guide future studies and applications for extramural funding. Research encompassing the mentored phase of this proposal will be conducted under the mentorship of Dr. Craig B. Thompson at Memorial Sloan-Kettering Cancer Center (MSKCC). Dr. Thompson is currently President and CEO of MSKCC, a member of the National Academy of Sciences and a leading expert in cancer biology. MSKCC has a highly collaborative atmosphere, state-of-the-art facilities and access to the resources of both Weill Cornell Medical College and The Rockefeller University. The plethora of resources and collaboration opportunities available will allow for growth as a scientist while completing the research outlined below. Cell growth and proliferation are highly coordinated processes stimulated by mitogenic signals and fueled by available nutrients. Broadly defined, mitogenic signals result from receptor stimulation by growth factors, cytokines, etc. or from aberrant activation of signaling pathways downstream of oncogenes or loss of tumor suppressors. MicroRNAs are small non-coding RNAs that regulate fundamental cellular processes, including cancer-associated mitogenic signaling. Among the first observed links between cancer-associated signaling and microRNAs was our discovery that Ars2, a protein necessary for expression of a large sub-set of microRNAs, is regulated by mitogenic signals. Recently we provided an additional link by demonstrating that mitogenic and metabolic signals enhance microRNA function, likely by promoting the assembly of the RNA- induced silencing complex (RISC), a protein-RNA complex required for microRNA activity. These studies, along with preliminary data contained in this grant application, suggest that microRNA biogenesis and function are enhanced by signals that promote cell growth and proliferation. The major goals of proposed research are 1) determining cellular signaling and metabolic pathways that regulate nuclear microRNA biogenesis 2) defining factors required for mitogen-stimulated RISC assembly and microRNA activation, and 3) determining the signaling and metabolic pathways affected by activation of microRNAs that are highly expressed in quiescent cells. To accomplish these goals, in vitro manipulation of normal and cancer cells will be performed to elucidate mechanisms underlying regulation of nuclear microRNA biogenesis, RISC assembly and microRNA activation. Additional studies will examine the ability of microRNAs that are highly expressed in quiescent cells to regulate cancer-associated signaling and cellular metabolism. Successful completion of research proposed to meet these goals will provide a novel framework in which the interrelationship of microRNA and cancer biology will be assessed.